1. Field of the Invention
The present invention relates to improvements in apparatus for aspirating liquids, e.g. biological specimens, from small vessels, e.g. test tubes or vials, for processing. More specifically, this invention relates to improvements in apparatus for sensing contact between the tip of a liquid-aspiration probe and the bottom of a vessel from which liquid is to be aspirated. The invention is particularly useful in automated hematology instruments for extracting the last useful blood sample volume from test tubes and the like for analysis.
2. The Prior Art
In conducting tests on biological liquids, e.g., blood, it is common to employ automated instruments to extract liquid specimens from small vessels, such as test tubes and the like. Such instruments typically include a movably-mounted sample aspiration probe that is adapted to be driven downwardly into the interior of a vertically-oriented test tube to a position in which the aspiration tip is below the level of sample liquid in the tube. In cases where the liquid sample is relatively small, such as a blood sample drawn from an infant, it is often necessary to advance the probe tip into near or virtual contact with the tube bottom in order to aspirate a sufficient volume of sample for testing. Since sample tubes can vary significantly in size, it can be problematic to consistently achieve probe tip/tube bottom contact without risking breakage of the tube. This problem is exacerbated when the probe tip is sharpened to puncture a rubber seal atop the tube.
In the commonly assigned U.S. Pat. No. 6,363,802 to Grippo et al., different types of apparatus are disclosed for maintaining contact between an aspiration probe tip and a tube bottom during a liquid aspiration process. In one embodiment, the tube is supported on a horizontal platform that is vertically movable and spring-biased towards a nominal rest position. As the aspiration probe is driven downwardly by a stepper motor or the like, the eventual contact between the probe tip and the tube bottom exerts a downward force on the tube-supporting platform, such force acting to physically displace the platform from its rest position. This displacement of the platform is directly sensed by a photo-electric sensor or the like, and an output signal from the sensor serves, through suitable circuitry, to arrest further operation of the stepper motor and to maintain the probe tip in contact with the tube bottom throughout the liquid aspiration. In a second embodiment, rather than supporting the tube on a movable platform, the tube is positioned on a fixed platform, and the forcible interaction between the probe tip and tube bottom is indirectly sensed by monitoring the back-emf in a winding of a D.C. motor used to advance the aspiration probe into the tube. While both of these embodiments can readily accomplish their intended purpose of sensing contact between the probe tip and container bottom, each may be problematic from different standpoints. For example, the movable platform approach tends to be relatively complex from a mechanical standpoint, and while the back-emf-sensing approach is mechanically simple, it can be relatively imprecise and less reflective of the actual force being applied between the probe tip and the container bottom.